1. Field of the Invention
The invention relates generally to the field of frequency conversion devices and in particular to gallium arsenide (GaAs) monolithic microwave integrated circuits (MMICs) therefor. More specifically, it relates to a low cost monolithic GaAs upconverter chip of the type used in a set-top cable television receiver.
2. Description of Related Art
Developments in GaAs monolithic microwave integrated circuit technology have made monolithic implementation of high frequency analog systems, such as cable television receivers and other radio receivers, the focus of considerable interest. The motivation for GaAs monolithic implementations is that such integration can lead to a considerable reduction in manufacturing cost and, sometimes, a performance improvement as well.
Reports of GaAs television tuner circuits appear in the literature. See, for example, Ducourant et al., "A 3 Chip Double Conversion TV Tuner System with 70 dB Image Rejection", Proc. 1989 IEEE Symposium on Microwave and Millimeter-Wave Monolithic Circuits; and, Mizukami et al., "A High Quality GaAs IC Tuner for TV/VCR Receivers", IEEE Trans on Consumer Electronics, August, 1988. These and other known circuits, however, suffer from various problems which diminish their performance and/or increase their cost relative to discrete component implementations of such devices as are in commercial use. Such problems include, for example: use of high-cost IC packaging technologies, requirements for too many off-chip components, demanding test procedures, excessive noise and/or distortion, and expensive fabrication technologies. These and other factors combine to preclude, so far, the development of effective, commercially advantageous realizations of GaAs tuner chips.
Four important factors which determine the ultimate manufacturing cost of any electronic system including a custom monolithic chip are: the number of external parts beyond the chip required to complete the system, the packaging cost of each chip, the testing cost (time/test), and the manufacturing yield (fraction of chips which can be sold, usually inversely proportional to the chip die area). Thus, a designer of a system which includes a custom monolithic chip will usually attempt to select a circuit topology that can be realized using the lowest cost chip package and the smallest number of off-chip components. In addition, since large value passive components, such as the inductors and capacitors conventionally used for filtering, can occupy a large fraction of the chip die area, circuit topologies which do not require such components are preferred in monolithic implementations where die area may significantly affect the yield and hence the cost of the chip. Finally, reduction of testing time is always useful in reducing the cost of any monolithic chip.
Upconverters, and frequency converters generally, typically include three basic functional blocks: an RF amplifier to amplify the RF input signal, a local oscillator (LO) to generate a LO signal, and a mixer to combine the RF input and LO signals to generate an intermediate frequency (IF) signal. With regard to RF input amplification and LO signal generation, prior art frequency conversion systems employ a variety of standard circuit topologies to implement these functions, the selection of which is typically determined merely by interfacing requirements for the intended application and the mixer topology used in the system. On the other hand, choice of a particular mixer topology is often critically important and may profoundly influence the ultimate performance of a frequency conversion system. Certain mixer circuits have particular advantages in terms of distortion, noise, and carrier suppression.
A class of mixers known as double balanced mixers exhibits excellent carrier suppression and low second order distortion. While good carrier suppression can also be achieved in non-double balanced mixers by the use of filters, such filters require large value inductors and capacitors which consume excess chip area. Thus, the double balanced mixer is well suited for monolithic integration since large area capacitors and inductors can be avoided.
The Gilbert type mixer, first described by Gilbert in 1969, is a double balanced mixer found in many integrated circuits and is well suited for use in a GaAs monolithic frequency converter. Although the Gilbert type mixer has excellent carrier suppression and low second order distortion, it suffers from high noise figure.
Mixer circuits which generate a product of input signals inherently produce an output signal containing a combination of two signals, an IF output signal and an IF image signal, both falling at the IF frequency. The IF output signal results from frequency translating the RF input signal to the IF frequency, and the IF image signal results from frequency translating the image signal to the IF frequency. The relationships among the IF, RF, LO and image frequencies for an upconverter are as follows:
FREQ(IF)=FREQ(LO)-FREQ(RF) PA1 FREQ(IF)=FREQ(image)-FREQ(LO)
Thus, there are signals of two different frequencies at the input of the mixer (for a given LO frequency) that will become frequency translated to the IF frequency, the RF frequency and the image frequency.
It has been discovered that a major source of noise in the Gilbert type mixer originates from input noise at the image frequency, and that such noise can be substantially eliminated in the manner described herein. Gilbert type mixers comprise two tightly intertwined stages, (1) an RF amplifier stage which amplifies the RF input signal, and (2) a chopper (or mixer) stage which mixes the amplified RF input signal with a LO signal to generate the IF signal. Since all devices in electronic circuits inherently produce broadband noise, the RF amplifier stage will generate input noise to the chopper stage, and such noise includes components at both the RF and the image frequencies. It has been discovered that by filtering the input noise at the image frequency before it is frequency translated to the IF frequency, and thus unremovable by filtration, the overall noise figure of the mixer described herein can be substantially improved.
Conventionally, in non-differential circuits, an image-rejection filter placed between the output of the RF amplifier and the input to the mixer will attenuate noise at the image frequency presented to the input of the mixer. This prevents noise at the image frequency from contributing to and increasing noise at the IF frequency. Prior art Gilbert type mixers have not used such image-rejection filters because the output of the RF amplifier stage could not be separated from the chopper (mixer) stage, and thus a filter could not usefully be placed between the RF amplifier and the mixer. Placing the image-rejection filter at the input of the RF amplifier (rather than its output) will not have the desired effect since the RF amplifier itself generates noise at the image frequency. See, N. Scheinberg et al., "A high-performance, miniaturized X-band active mixer for DBS receiver application with on-chip IF noise filter", IEEE Trans on Microwave Theory and Techniques, September 1990, pp. 1249-1251.
Another important function of a receiver is to provide automatic gain control (AGC). The function of an AGC circuit is to adjust the gain of the receiver so that the IF output signal level remains relatively constant despite variations in the RF input signal level. This is typically accomplished by varying the gain of the RF input amplifier in response to changes in the RF input signal level.
A major problem with prior art AGC circuits is that they often introduce unwanted distortion. Conventionally, the AGC function was added to a Gilbert type mixer by replacing the source degeneration resistors with an AGC FET (MOSFET, MESFET, GASFET, etc.). This AGC FET acted as a voltage-controlled resistor whose drain-to-source resistance R.sub.DS varied with the voltage V.sub.G applied to the gate of the AGC FET. Since the gain of such a Gilbert type mixer depends on R.sub.DS, the voltage V.sub.G at the gate of the AGC FET could be used to control the gain of the mixer.
A problem with these prior art AGC circuits is that the AGC FET resistance R.sub.DS is not only dependent on the gate voltage V.sub.G, but also on the RF input voltage to the mixer and this latter dependence leads to increased distortion. Thus, there is a need for a low distortion AGC circuit compatible with a Gilbert type mixer but without the conventionally associated distortion problems.
A third problem associated with high frequency analog chips such as CATV upconverters is that such circuits have typically required specialized RF packages since the large pin inductances associated with standard plastic dual inline (DIP) packages would substantially degrade circuit performance. The prior art contains a number of monolithic GaAs CATV-related upconverter and downconverter circuits, most of which are packaged in conventional high cost RF packages. The cost of these packages can exceed the cost of the remainder of the chip, making many applications simply too costly (relative to existing discrete component circuits) to be commercially feasible as integrated circuits. There is thus a need for circuit topologies with reduced sensitivity to the parasitic inductances of low cost DIP packages.
Testing also represents a significant fraction of the manufacturing cost of any integrated circuit. GaAs integrated circuits have been known to exhibit anomalous behavior associated with back-gate transient swings, often governed by very long time constants, which may result in very long test times at significant cost.
Current theories regarding the back-gate effects in monolithic GaAs metal semiconductor field effect transistors (MESFETs) are described in the articles: "An Accurate MESFET Model for Linear and Microwave Circuit Design" by Scheinberg et al., IEEE JSSC, April 1989; "Carrier Injection and Backgating Effect in GaAs MESFET's" by Lee et al., IEEE EDL, April 1982; and "An Improved GaAs MESFET Equivalent Circuit Model for Analog Integrated Circuit Applications" by Larson, IEEE JSSC, August 1987, all of which are incorporated herein by reference.